1. Field of the Invention
The present invention concerns a method for automatic state control, inspection, cleaning and/or surface treatment of structures, especially measuring the thickness of plate constructions and pipes by transmitting ultrasound signals from at least one transmitter and receiving the ultrasound signals from at least one receiver, where the transmitter and receiver are mounted to a self-propelled, remote controlled unit, and where a liquid membrane is present between the transmitter and the surface of the structure.
2. Description of the Prior Art
When, for example, a ship is to be classified, it is necessary to collect information concerning the remaining thickness of the plates in order to calculate the strength of the hull and to certify the ship for continued operation. Furthermore, for the shipowners it is desirable to obtain knowledge of the condition and the progress of the ship in order to perform overhauling and preventive maintenance in the most efficient way, optionally to make a strategic decision of taking the ship out of operation and sell it as scrap iron, at the correct point in time.
Today, the thickness of marine steel constructions such as ships and drilling plateforms are measured by manual spot measurements (sampling test according to a certain system) which are conducted from the interior or from the exterior by a diver with a hand held measuring apparatus when the ship is in harbor. Interior measurements may also be performed in empty tanks during transit.
The prior method has obvious disadvantages. Full coverage of the hull of the ship is not achieved, and selection of the position of the measurement spot is a subjective judgment. The strength of the reflected ultrasound signal is highly dependent upon the condition of the rear wall, and in areas having corrosion pitting (a common and dangerous phenomenon), the distance between areas having full remaining thickness and areas corroded close to the danger point will be small. Between these adjacent extremes of thickness, the rear wall will necessarily slant. The transmitted signal will thus mainly be reflected to the side of the receiver which only receives a weak signal. The inspector is taught to reject weak measurements in accordance with certain criteria. By moving the measuring probe a few centimeters, the inspector will register a strong signal from an almost flat rear wall with full plate thickness. The ship may then pass the control, and the shipowner has got poor basis for decisions concerning preventive maintenance on the ship. At a later docking, the actual situation will become clear, and the subsequent remedial work will be alot more expensive than planned.
From EP 0 139 867, is known a self-propelled unit having transmitters and receivers for ultrasound signals for determining the material quality of a body. This known system is not suited for elimination of single measurements with weak signals.